A light receiving device which is utilized in an optical communication system is required to be of a high speed response, a high sensitivity, etc. An avalanche photodiode which has the highest sensitivity in receiving a light having a bit rate of approximately several Gb/s has been proposed. Such an avalanche photodiode is provided with an internal gain based on the avalanche multiplication, thereby meeting the requirements of the properties set forth above.
In the avalanche photodiode, it is required to select a material system having a small avalanche noise, so that a sensitivity is improved at a high speed response, because the sensitivity-characteristic depends largely on an avalanche multiplication process, in which carriers are ionized in accordance with the scattering thereof based on the acquisition of an energy approximately 1.5 times an energy gap in an electric field. More particularly, it is necessary to provide a large difference between ion densities .alpha. and .beta. of electron and hole in an avalanche multiplication layer of the avalanche photodiode, where the ion densities .alpha. and .beta. are defined by frequencies of the ionization of electrons and holes. If the ion density .alpha. is equal to the ion density .beta. (.alpha.=.beta.), wherein electrons and holes are excited with an equal provability, the chain reaction of ionization continues, so that a large random shot noise is produced.
A spectrum density i.sub.NS of a shot noise is defined by below equations, ##EQU1## where q is a unit charge, I is an optical current, and M is a multiplication factor.
As understood from the above, a ratio (.alpha./.beta.) of the electron ion density .alpha. and the hole ion density .beta. is defined to be a figure of merit, on which a noise characteristic of an avalanche photodiode depends.
Where Si is adopted for an avalanche photodiode of 0.8 .mu.m wavelength band, the ratio (.alpha./.beta.) is approximately 20, so that a high performance is obtained therein. On the contrary, an avalanche photodiode which is used at a wavelength band of 1.55 .mu.m has the ratio (.alpha./.beta.) of approximately 2, so that the reduction of a noise level is limited to some extent, because the avalanche photodiode includes a multiplication layer of InP.
Avalanche photodiodes which have been described on pages 467 and 468 of "Electronics Letters, 5th June 1980, Vol. 16, No.12" and on pages 597 to 599 of "Appl. Phys. Lett. 47(6), 15 Sept. 1985" include an electron multiplication layer of InGaAs/InAlAs superlattice, wherein the ratio (.alpha./.beta.) has been proposed to be increased in accordance with a large discontinuity (approximately 0.5 eV) of conduction band between InGaAs and InAlAs. In other words, the ratio (.alpha./.beta.) is increased in accordance with the contribution of the superlattice band-discontinuity to an ionization energy.
In an avalanche photodiode including a multiplication layer of superlattice, ion densities .alpha. and .beta. of the multiplication layer are defined by a below equation, ##EQU2## where E.sub.i,th is an ionization energy, E.sub.R is a phonon scattering energy, .lambda. is a mean free path in a phonon scattering process, and E is an electric field.
In the above equation, EQU E.sub.i,th =E.sub.i.sup.b.sub.,th -.DELTA.E.sub.c (for .alpha.) EQU E.sub.i,th =E.sub.i.sup.b.sub.,th -.DELTA.E.sub.v (for .beta.)
where .DELTA. E.sub.c and .DELTA. E.sub.v are band discontinuities with respect to a conduction band and a valence band, respectively, provided that E.sub.i.sup.b.sub.,th is an ionization energy of a bulk.
As understood from the above equation, the ion densities .alpha. and .beta. are largely affected by the change of an ionization energy, because the ionization energy is included in the exponent of the equation. Therefore, it is required that the band discontinuity .DELTA.E.sub.c is largely greater or less than the band discontinuity .DELTA. E.sub.v (.DELTA.E.sub.c &gt;&gt;E.sub.v or .DELTA.E.sub.c &lt;&lt;.DELTA.E.sub.v), thereby increasing the ratio (.alpha./.beta.). In considering a response characteristic, carriers which are avalanche-multiplied are desired to be of electrons each having a high mobility. In accordance with the assumption of ".alpha.&gt;.beta.", the band discontinuity .DELTA. E.sub.c should be largely greater than the band discontinuity .DELTA. E.sub.v, which is also desired to be zero (.DELTA.E.sub.v =0) to avoid the deterioration of a response characteristic due to holes, because the band discontinuity .DELTA. E.sub.v functions badly as a carrier trap for holes.
However, the proposed avalanche photodiodes have a disadvantage in that there is a difficulty in increasing the ratio (.alpha./.beta.) at the electron multiplication layer, because a discontinuity, which is approximately 0.2 ev, exsists even in the valence band at the InGaAs/InAlAs superlattice. In addition, the valence band discontinuity of 0.2 ev results in the deterioration of a response characteristic, because it becomes a trap for holes.